Combined hyperbolic and parabolic double reflector



Jan. 23, 1968 V K. s KELLEHER 3,

COMBINED HYPERBOLIC AND PARABOLIC DOUBLE REFLECTOR Filed Dec. 21, 1961 2Sheets-Sheet 1 FIG. I

PRIOR ART IN VEN TOR.

KENNETH s. KELLEHER ATTORNEY Jan. 23, 1968 K. s. KQELLEHER 20 COMBINEDHYPERBOLIC AND PARABOLIC DOUBLE REFLECTOR Filed Dec. 21, 1961v r 2Sheets-Sheet 2 INVENTOR. KENNETH S. KELLEHER A TTORN E Y United StatesPatent ()fiflce 3,365,720 Patented Jan. 23, 1958 3,365,720 COMBINEDHYPERBQLIC AND PARABOLIC DOUBLE REFLECTOR Kenneth S. Kelleher,Alexandria, Va., assignor to Keltec Industries, Inc a corporation ofVirginia Filed Dec. 21, 1961, Ser. No. 161,018 12 Ciairns. (Cl. 343-837)The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; U.S.C. 2457).

The present invention relates generally to microwave technology, and,more particularly, to a compact, lownoise, microwave antenna assembly.

In the microwave antenna art, some of the problems which constantlyarise are: (1) noise caused by heat energy from the earth; (2) undesiredside lobes which appear in the radiation pattern; and (3) feed blockingof the reflected waves. Feed blocking may be explained as follows: If asymmetrical paraboloid of revolution is used as the reflector, and afeed born or the like is provided at the focus of the parabola, thenthere will be blocking of some of the waves which are reflected from theparaboloid of revolution and this disadvantageously affects theradiation pattern, especially since this blocking occurs in the centerof the wave path. There have been attempts in the past to correct thiscondition.

One type of low-noise antenna for microwave transmission is the shieldedhorn of the type illustrated in FIGURE 1 and which is disclosed byKenneth S. Kelleher on pages 12-14 of the Antenna Engineering Handbookpublished by the McGraW-Hill Book Company, Inc. This is a hornparaboloid reflector system, also called a shielded horn, and is a verysuccessful system which provides a partial solution to problems 1) and(2), and which is useful in reducing wide-angle radiation. Anotherbeneficial feature of this system is that the feed point FP at the focusof the parabola R in no way interferes with the reflected waves whichare directed through radiation aperture A from the parabolic surface, sothat there is no feed blocking.

However, this shielded horn is subject to the disadvantage of beingfairly large in size. Such a horn must be at least as great in length asthe aperture dimensions. Although a wide flare angle may be used in anattempt to reduce the length of the horn, when this is done, poorpattern characteristics follow. Therefore, while the lownoisecharacteristic of such an antenna assembly is desirable, its large sizepresents a disadvantage.

With these defects of the prior art in mind, it is a main object of thepresent invention to reduce drastically the size of present low-noiseantenna systems.

A further object of this invention is to provide an antenna assembly ofthe type described which provides a relatively low level of noise, yetis compact.

Another object is to provide an antenna assembly which substantiallyreduces noise caused by heat energy from the earth.

Still another object is to provide an antenna assembly which effectivelylessens spill over and undesired side lobes Which detract from theradiation pattern.

Yet a further object of the invention is to substantially eliminate feedblocking.

These objects and others ancillary thereto are accomplished according topreferred embodiments of the invention, wherein two reflectors are usedwhich are situated so that there will be substantially no blocking ofthe waves between the feed source and the finally directed wave path.This is particularly important in the center of the path and in someinstances blocking may be tolerated about the edges if this is needed toobtain certain desired characteristics.

The feed source is disposed on one reflector and directed by a horn orthe like toward the other reflector. The second reflector reflects thewaves back in such a manner that they are reflected from the firstreflector in a parallel arrangement so that the antenna may be directedor aimed as desired.

For example, instead of using a reflector which forms a symmetricalparabola with respect to the feed point and the axis of this curve, onlya portion of one side of the parabola is used so as to eliminate feedblocking. Another reflector, which is a segment of a hyperbola, facesthis arcuate portion of the parabolic reflector and in an extreme casemay be planar.

If the focus of the parabolic reflector is assumed to be one of thefocal points of a hyperbola, and the feed for the antenna is assumed tobe the other of the focal points of a hyperbola, then the combination ofthe parabola and a hyperbola, constructed from these two focal points,will form the reflecting surfaces. These surfaces will then be such thatthe feed or lower focal point of the hypenbola, which is preferablydisposed on the parabola, will have its Waves reflected from thehyperbolic reflector to the parabolic reflector where they will appearto have emanated from the focus of the parabola which is now the virtualfeed. The reflected waves are reflected from the parabolic reflector andin a direction parallel to the axis of the parabola and through theradiation aperture formed between the edges of the parabolic and thehyperbolic reflectors.

The reflectors may be hyperbolic and parabolic cylinders or paraboloidsand hyperboloids of revolution. The reflectors may also be planarsurfaces in combination with parabolic cylinders or paraboloids ofrevolution. In the case of cylindrical reflectors it is preferable tohave a line feed source, while in the case of surfaces of revolution, 21point feed source is preferable.

Additional objects and advantages of the present invention will becomeapparent upon consideration of the following description when taken inconjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of the shielded horn antenna system whichis known in the prior art.

FIGURE 2 is a digrammatic view illustrating the theory of the parabolicand hyperbolic reflectors, with the feed disposed along the length ofthe parabolic reflector.

FIGURE 3 is a perspective view of an antenna constructed in accordancewith FIGURE 2 and of the type wherein the reflectors are hyperbolic andparabolic cyl- .inders.

FIGURE 4 is a front elevational view illustrating the mountingarrangement for the antenna system.

FIGURE 5 is a diagrammatic view of the geometrical arrangement of thereflectors wherein the hyperbolic surface has been replaced by a planarsurface and the feed is at the edge of the parabolic surface.

FIGURE 6 is a diagrammatic view similar to FIG- URE 5, but wherein thefeed is disposed between the two reflectors.

FIGURE 7 is a perspective view of the embodiment of FIGURE 6.

FIGURE 8 is a perspective view of a dome-like assembly, whereinparaboloids and hyperboloids of revolution are used as reflectors, andtaken looking into the radiation aperture.

In the study, analysis, and design of antennas, transmitting andreceiving antennas are treated alike. That is, both types are consideredas transmitting antennas. It should therefore be noted that although theanalysis I which follows may seem to indicate that the antenna of thepresent invention is a transmitting antenna, it is primarily a receivingantenna, albeit usable for transmitting purposes with excellent results.In accordance with the above, the feed source, or feed, which isassociated with the antenna, means a feed for the antenna in the case oftransmitting, and a feed for the receiver in the case of receiving. Thisconvention is also followed in the claims. However, in severalinstances, which should be immediately apparent, this convention has notbeen followed, and the waves are considered as travelling toward, ratherthan away from, the antenna.

The reflectors should be such that, considering them as a receivingantenna for the moment, the parallel waves entering the aperture will bereflected from the first surface on which the feed source is located anddirected toward the second reflector. The wavefront of the wavesreflected from the second reflector should be spherical and focus to thefeed source on the first reflector. Also, to prevent feed blocking, thefirst reflector should be asymmetrical.

Thus, the problem may be set up as follows using the relationshipstaught by Kenneth S. Kelleher in Relations Concerning Wave Fronts andReflectors in the June 1950, issue of Journal of Applied Physics, volume21, No. 6, pages 573576.

Given the primary asymmetric reflector R and the incident plane wave X,fined the reflected wavefront Y.

Y (u, a, v)

Now, from the reflected wavefront Y, find the second reflector surface Rwhich produces a reflected wave focusing to a point on the primaryasymmetrical reflector.

for X =0, origin at focal point Hi 2[0+r-n Since 5 is perpendicular to YC Y R: Y+ TE where Y Y l Y Yvi and C is a constant.

Using such formulas, it can be calculated that the following curves maybe used to complement each other:

parabola=hyperbola catenary catenary complement circle=cardioid thelatter of which is known as the Ziess Cardioid and has been described byZernike.

With more particular reference to the drawings, FIG- URE 2 is a diagramindicating the geometrical arrangement of one form of the assembly,which may be considered to be a cross section of an antenna system. In atypical reflector arrangement, a parabolic reflector is provided, whichmay be considered as curve P. In this arrangement a point feed sourcewould be located at the 7 afocal point P of the parabola. This parabolahas an axis X which is parallel to the radiating direction, and adirectrix (not shown) which is required in order to construct theparabola so that all points therealong are equidistant from focus F andlines which are at right angles to the directrix.

In order to reduce the size of this parabolic reflector P, a hyperbola His constructed from upper and lower focal points F and S, which are thefocus of the parabola P and the feed point, respectively. A constructionline I is drawn between these focal points F and S, and a perpendicularbisector B is constructed. From this given information, the hyperbola Hmay be constructed. The hyperbola H intersects parabola P at point e.

Now for the preferred embodiment which yields optimum results insofar asgain is concerned, the line ab which defines the plane of the radiatingaperture, should be at right angles to the parabola axis X and passthrough its focus F. At the same time, an imaginary line adjoining a andc of hyperbola H should be disposed parallel to axis X. With such aconstruction, the waves emanating therefrom will be directional andparallel to axis X.

However, it should be noted that in order to obtain particularcharacteristics it may become necessary to deviate somewhat from thisoptimal arrangement. For example, imaginary line ac may be disposed atan angle with respect to axis X in such manner that there is someblocking of waves. This may be readily tolerated up to about 10%blocking. Also, since this blocking occurs at the side or edge of thewave path, it is not as objectionable as the type of feed blockingmentioned hereinbefore which is at the center of the wave path.

Points 1 and 2 along hyperbolic reflector H have been chosen at random.Lines V1 and V2 are now drawn from focus F to points 1 and 2. If we nowassume that point S is a feed source and feed microwaves thereto, a waveA1 will be radiated as indicated and will arrive at hyperbolic reflectorH at point 1 and be reflected therefrom along the line R1. This willthen be reflected in a direction parallel to axis X along line W1. Asimilar result will be provided by a radiated wave A2 which is reflectedas indicated by line R2 and is then emitted from the radiating apertureas indicated by line W2.

From the foregoing it may be seen that although the actual point ofradiation is disposed along the parabolic reflector P between points 0and b, the waves which are reflected from the hyperbolic reflector H, R1and R2, will actually be extensions of and coincident with lines V1 andV2. It will thus appear that these waves have originally emanated fromfocus F of parabola P which may be considered the virtual feed source.Thus, the system has all of the attributes of the typical parabolicreflector P, but is obviously drastically reduced in size.

This system will also have the low-noise characteristic provided by theshielded horn illustrated in FIGURE 1, but will be of much smaller size.

With more particular reference to FIGURES 3 and 4, a practical device isillustrated which is constructed in accordance with FIGURE 2. A curvedplate of conducting sheet metal 10 is provided which conforms to aparabola so as to define a parabolic cylinder, and which may beconsidered as the arcuate section of curve P between points 11 and 0.Panel 12 is constructed of similar material and is a hyperbolic cylinderwhich may be con sidered as the curve H, that is a hyperbola, betweenthe points a and 0. End panels 14 and 16, which are parallel to eachother, are provided to join and support the edges of panels 10 and 12,and the edges of panels 10, 12', 14 and 16, are disposed in a plane anddefine a radiating aperture 11. The abutting edges of panels 10 and 12are contiguous so that a closed four-sided reflector is provided. A linesource of radiation is provided in the form of feed 18 which is fed by atransmission line 20 from a suitable microwave generating or receivingapparatus 22, which is well known in this art.

This microwave assembly is supported by means of a yoke 24, having amotor 26 joined to one leg thereof and a drive shaft 28 attached topanel 16, so that upon rotation of motor 26, shaft 28 will turn andforce the entire assembly to rotate in a direction about a transverseaxis which is shown as being coincident with feed source 18. The otherleg of the yoke has a bearing 32 connected thereto and in which a shaft30 is journalled. The other end of this shaft is rigidly fastened topanel 14, so that the entire assembly may rotate within the yoke 24.

A conduit 23 is provided through an opening passing through shaft 30, tohouse transmission line 20.

The yoke itself is mounted upon a support 34 having a motor 36 mountedthereon and which is connected with and rotates yoke 24 by means of amotor drive shaft 38 which is connected to yoke 24. Thus, by controllingmotors 26 and 36 the directional antenna assembly may be controlled soas to emit the waves in any desired direction.

With more particular reference to FIGURE 5, another embodiment isillustrated which, in construction and operation is very similar to thatof FIGURES 2-4, but with certain specific differences. For example, inthis embodiment the feed source S is disposed at the edge of parabola Pat point b. The hyperbola H of FIG. 2 has become, in this case, planaror a straight line H. With such a construction point a is equidistantfrom points F and [7.

FIGURE 6 shows an embodiment which is similar to FIGURE 5, but whereinthe feed source S is disposed between the curves P and H. Furthermore,the two adjacent surfaces of curves P and H are not contiguous, but arespaced from each other as may be seen by the space between points on theparabola P and c on curve H. The construction of the curves in thisexample is the same as those set forth in the other examples. However,it should be noted that an imaginary line joining points a and c isparallel to axis X so that the radiating aperture between points a and bdefines the extreme limits of the radiation surfaces which will radiatein the intended radiating direction. All of the constructional elementsin these figures which are similar to those of FIGURE 2 are indicated byidentical reference characters.

In the perspective view of this embodiment illustrated in FIGURE 7, theparabolic reflector 40 and the extreme hyperbolic or planar reflector 42do not touch each other but are spaced and are supported by end panels44 and 46 and any other suitable reenforcing structure which may berequired. The end panels 44 and 46 rigidity the ends of the reflectors40 and 42. In this instance also the feed is a line source 50 which ismounted so as to be disposed inwardly of the surface of parabolicreflector 40.

In the embodiment illustrated in FIGURE 8 the parabolic and hyperbolicreflectors are a paraboloid of revolution 52 and a hyperboloid 54 whichare joined together at their adjacent ends to form a contiguous surfaceand are provided with supporting end panels 56 and 58. In this instancea point source of radiation 60 is provided, and a reflected wave isindicated with the same indicia, as was used in FIGURES 1, and 6, toindicate the reflection of a wave within the assembly and thenexternally of the horn in the desired direction.

Thus, by means of this embodiment of the present invention the realsource, which is mounted in close proximity to the radiation aperture ofa shielded horn, can produce a virtual source for waves reflected fromthe hyperbola. This virtual source is a great distance from theradiation aperture and matches in every respect the characteristics of areal source placed at this distance from the aperture. In general, thefeed point for introducing wave guide energy into the antenna assemblywill be positioned at some point along the parabolic surface which isequidistant from the two side panels.

As was mentioned above, although it is not a necessity that line ab beat right angles to axis X and pass through focus F, such a constructionwill provide optimum condition will provide optimum conditions for highgain purposes. The -feed may be provided by a directional horn to directthe waves toward the intended reflector, and this will reduce spill overto a large extent.

The hereinabove desc-ri-bed antenna system is useful in the microwaveregion which is in the range of about 10 or 10 cycles per second. Atypical size for a practical embodiment for FIGURE 8 would be ten feetby ten feet, with a depth of eight feet. From these dimensions it shouldbe readily realized that the size of such an antenna is quitedrastically reduced from that which would be required for the typicaltype of parabolic reflector or shielded horn which has been used in thepast.

Any type of suitable control may be provided to accurate-1y controlmovement of the motor shafts so that the antenna assembly may becritically aimed, if desired. Such motors and controls therefor arealready known per se and therefore will not be described in detail.

Each of the arrangements set forth in FIGURES 2, 5, and 6, may beconstructed with the reflectors as parabolic and hyperbolic cylinders asshown in FIGURE 3, or as paraboloids and hyperboloids of revolution asshown in FIGURE 8. However, by using the hereinabove describedinvention, the reflectors may be provided with other configurationswhich will provide similar results. Such configurations may becalculated by using the above formulas. The antenna of the presentinvention drastically reduces heat energy noise from the earth and spillover, as well as feed blocking. Accordingly, the low-noisecharacteristic of this antenna is extremely favorable.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations, and the same are intended to r be comprehended within themeaning and range of equivalents of the appended claims.

What is claimed is:

1. A microwave antenna assembly for radiating microwaves in an intendeddirection comprising, in combination:

(a) a first reflector defining a hyper-bolic arc in cross section;

(b) a second reflector adjacent said first reflector and defining aparabolic arc in cross section whose focus is disposed exteriorly ofsaid assembly; and

(c) a radiation feed source disposed for radiating waves toward saidfirst reflector; said reflectors together defining a radiation aperturewhich is in a plane disposed at right angles to the intended radiationdirection, said hyperbolic are having the focus of said parabolic are asone focal point and said feed source as another focal point, saidreflectors being arranged so that waves from said source directed towardsaid first reflector are reflected toward said second reflector alonglines coincident with imaginary lines between the focus and said secondreflector, whereby all waves reflected from said second reflector willbe directed in a direction at right angles to the plane of saidradiation aperture.

2. An assembly as defined in claim 1, wherein the focus of the parabolicarc of said second reflector is in the same plane as said radiationaperture.

3. An assembly as defined in claim 2, wherein said first reflector is ahyperbolic cylinder and said second reflector is a parabolic cylinder.

4. An assembly as defined in claim 3, wherein said source extends forthe length of said reflectors.

5. An assembly as defined in claim 1, wherein said source is a pointsource.

6. An assembly as defined in claim 2, wherein said reflectors form acontiguous surface.

7. An assembly as defined in claim 1, wherein said first reflector is ahyperboloid of revolution and said second reflector is a paraboloid ofrevolution.

8. An assembly as defined in claim 7, wherein said the edge of saidfirst reflector defining in part said radiasource is a point source ofradiation. tion aperture, and the edge of said second reflector ad- 9.An assembly as defined in claim 2, wherein said jacent said firstreflector, is disposed in the intended radiasource is mounted on saidsecond reflector. tion direction.

10. A directional, low-noise microwave antenna assem- 12. In a low-noisemicrowave antenna, the combinably, comprising, in combination: tionwhich comprises:

(a) a first reflector defining a hyperbolic arc in cross (a) a parabolicreflector having an axis along which section; there is a focus; i (b) asecond reflector adjacent said first reflector and (b) a radiationsource spaced from said focus; and defining a parabolic arc in crosssection whose focus m (c) a hyperbolic reflector which is hyperbolicwith is disposed exteriorly of said assembly; respect to said source andsaid focus for directing (c) a radiation feed source disposed withinsaid assembly; and

(d) a pair of end frame panels disposed in planes parallel to theintended radiation direction of said assembly, said panels and saidreflectors together defining a radiation aperture which is disposed in aplane at right angles to the intended radiation direction and in thesame plane as the focus of the toward said parabolic reflector radiationdirected by said source against said hyperbolic reflector such that saidsource appears to said parabolic reflector, to emanate from said focusso that said parabolic reflector directs the radiation in a directionparallel to said axis, said hyperbolic reflector being locatedsubstantially outside of the radiation reflected by said parabolicreflector.

parabolic arc defining said second reflector, said 20 hyperbolic archaving the focus of said parabolic arc References Cited as one focalpoint and said feed source as another UNITED STATES PATENTS focal polnt,said reflectors being arran ed so that waves from said source directedtoward said first re- 2,579,140 12/1951 Qfawford 3439 14 X flector arereflected toward said second reflector 25 2,960,693 11/1960 rry 343 912X gong lines concideint with dimagiinary linels bebtweeg FOREIGN PATENTSt e ocus an sai secon re ector w ere y a q 577,939 6/1946 British. wavesreflected from said first reflector will event u- 1,128,952 9/1956French ally be directed in a direction at right angles to the plane ofsaid radiation aperture.

11. An assembly asdefined in claim 10, wherein said reflectors arearranged so that an imaginary line, between 30 ELI LIEBERMAN, PrimaryExaminer.

HERMAN K. SAALBACH, Examiner.

1. A MICROWAVE ANTENNA ASSEMBLY FOR RADIATING MICROWAVES IN AN INTENDEDDIRECTION COMPRISING, IN COMBINATION: (A) A FIRST REFLECTOR DEFINING AHYPERBOLIC ARC IN CROSS SECTION; (B) A SECOND REFLECTOR ADJACENT SAIDFIRST REFLECTOR AND DEFINING A PARABOLIC ARC IN CROSS SECTION WHOSEFOCUS IS DISPOSED EXTERIORLY OF SAID ASSEMBLY; AND (C) A RADIATION FEEDSOURCE DISPOSED FOR RADIATING WAVES TOWARD SAID FIRST REFLECTOR; SAIDREFLECTORS TOGETHER DEFINING A RADIATION APERTURE WHICH IS IN A PLANEDISPOSED AT RIGHT ANGLES TO THE INTENDED RADIATION DIRECTION, SAIDHYPERBOLIC ARC HAVING THE FOCUS OF SAID PARABOLIC ARC AS ONE FOCAL POINTAND SAID FEED SOURCE AS ANOTHER FOCAL POINT, SAID REFLECTORS BEINGARRANGED SO THAT WAVES FROM SAID SOURCE DIRECTED TOWARD SAID FIRSTREFLECTOR ARE REFLECTED TOWARD SAID SECOND REFLECTOR ALONG LINESCOINCIDENT WITH IMAGINARY LINES BETWEEN THE FOCUS AND SAID SECONDREFLECTOR, WHEREBY ALL WAVES REFLECTED FROM SAID SECOND REFLECTOR WILLBE DIRECTED IN A DIRECTION AT RIGHT ANGLES TO THE PLANE OF SAIDRADIATION APERTURE.